Monthly Fee Curiosity Question

So whenever someone mentions how much they pay for WoW they quote the base monthly cost $15 a lot. So I'm curious how many of you actually pay month to month, or subscribe for the 6 months at a time and save the $12. I've only ever done the 6 months at a time, and I'm like why not just sub instead of paying the monthly rate?

Is it budget thing? Is it want to see where the game goes after playing a month? Enlighten me, please.